I have an aspire v5 machine with intel graphics. The problem with it is that the function keys for brightness controll doesn't work under linux. On openrc I have used a /etc/local.d script which called a bash script but when moved to systemd I dont know how to do it proper.

If set to forking it is expected that the process configured with ExecStart= will call
fork() as part of its start-up. The parent process is expected to exit when start-up is
complete and all communication channels set up. The child continues to run as the main
daemon process. This is the behavior of traditional UNIX daemons.

Where is the setup of your child process?
Wouldn't this fit better:
Type=idle_________________fun2gen2